A High Finesse Cryogenic Sapphire Cavity for Ytterbium Optical Atomic Clocks

The short-term stability of virtually all optical atomic clocks is currently limited by the coherence time of the interrogation laser. Achieving laser stability well below the 10^-16 fractional frequency level will advance next-generation clocks for tests of fundamental physics, mapping Earth's geopotential, and redefining the SI second. Cryogenic optical cavities based on silicon have had some success realizing improved stability via reduced thermal noise effects.  Here, we report on development of a cryogenic sapphire optical cavity to stabilize the interrogation laser for NIST's ¹⁷¹Yb optical lattice clocks. Sapphire has an extremely low coefficient of thermal expansion of 10^-10 K^-1 at 4K, is transparent in the NIR and visible domains, and is extremely rigid, making it an ideal material for designing a highly stable reference cavity at cryogenic temperatures. We present progress towards achieving sub-10^-16 stability with a 4 K cryogenic sapphire cavity with finesse of 255,000.